1. Field of the Invention
The present invention relates to a disc driving device for driving a disc-like magnetic recording medium to record and reproduce information and more particularly to a disc driving device having a shift mechanism of a simple construction.
2. Description of the Prior Art
In a disc driving device for driving a disc-like magnetic recording medium (hereinafter referred to simply as "disc") to effect information recording and playback, the reduction in size of the device has been promoted with increase in density of information and reduction in size of the disc.
In a conventional disc driving device, as a carriage moving mechanism for moving a carriage which carries a magnetic head thereon intermittently in a radial direction of the disc, there have been both a steel belt type and a screw shaft type. The former type is disadvantageous in that the space factor is low in addition to troublesome operations required for mounting a belt and adjusting tension. For this reason, the number of shift mechanisms which adopt the latter screw shaft type has been increasing recently.
FIG. 15 shows an example of such a conventional screw shaft type carriage moving mechanism, in which the reference numeral 50 denotes a housing and numeral 51 denotes a stepping motor. A casing 52 of the stepping motor 51 is secured to the housing 50 with bolts 53. Within the casing 52, a coil 55 wound round a coil bobbin 54 and a comb teeth-like yoke 56 integral with the casing 52 are mounted as motor stator-side components, and a cylindrical permanent magnet 57 is mounted as a motor rotor.
Numeral 58 denotes a screw shaft having a spiral groove 58a formed on its outer peripheral surface. The screw shaft 58, with the permanent magnet 57 fitted thereon fixedly, is supported rotatably at its conical ends of a small diameter by bearings 59 and 60 comprising ball bearings. The bearing 59 is held by the housing 50 through a holder 62 to prevent its falling off. And it is urged toward the screw shaft 58 at all times by means of a spring 63 disposed within the holder 62. Consequently, the screw shaft 58 is urged toward the bearing 60 at all times. The bearing 60 is attached to an adjust screw 65 which is threadedly fitted in a support member 64 mounted to the stepping motor 51. The screw shaft 58 can be adjusted slightly in a thrust direction by turning the adjust screw 65.
Numeral 68 denotes a carriage which carries a writing/reading magnetic head (not shown) thereon and which is moved along guide rods 69. Numeral 70 denotes an engaging piece attached to the carriage 68, the engaging piece 70 having a conical tip which is in resilient engagement with the spiral groove 58a of the screw shaft 58. Under such an arrangement, as the screw shaft 58 is rotated forward and reverse by the stepping motor 51, the tip portion 70a of the engaging piece 70 fitted in the spiral groove 58a moves, so that the carriage 68 is reciprocated by distances corresponding to the amounts of rotation of the screw shaft 58 in approximately the same direction as the screw shaft along the guide rods 69.
In the above shift mechanism, the stepping motor 51 is mounted by utilization of lateral holes 50a, 50b and tapped holes 50c which are all formed in the housing 50. Two guide rods 69 are provided generally parallel to the screw shaft 58 of the stepping motor 51. Both ends of each guide rod 59 are attached to slots (not shown) formed in the housing 50, which slots are formed accurately as a reference plane for maintaining the height of the carriage constant. Consequently, the carriage 68 can be moved always along the plane formed by the two guide rods 69 regardless of in which position the tip portion 70a of the engaging piece 70 engages the spiral groove 58a of the screw shaft 58. Where the moving plane of the carriage 68 is thus defined by the two guide rods 69, the mounting accuracy of the screw shaft 58, namely, the mounting accuracy of the stepping motor 51, need not be so high as that of the guide rods 69, and even if it is relatively rough, a practically sufficient transfer accuracy is obtained. For this reason, usually the screw shaft 58 is loosely fitted in the lateral hole 50a formed in the housing 50 and the bearing 59 of the screw shaft 58 is fitted in the holder 62 which is press-fitted in the lateral hole 50b formed in a side wall of the housing 50, and a side wall of the casing 52 is mounted to the side wall of the housing 50 through bolts 53 in the tapped holes 50c. Before tightening the bolts 53, the position of the screw shaft 58 is adjusted by moving the body of the stepping motor 51, and thereafter the bolts are tightened and fixed.
Where the shift mechanism constructed as above is to be reduced in size and simplified in structure, it is considered extremely rational to omit one of the two guide rods 69, that is, constitute the shift mechanism by a single guide rod and a single screw shaft 58. In this case, it is necessary for the screw shaft 58 to have both a guide function and a feed function. But if a guide function is imparted to the screw shaft 58, there arises a problem in point of its mounting accuracy.
In this connection, for adjustment of the screw shaft 58, the shaft is loosely fitted in the lateral hole 50a formed in the housing 50 and is adjusted with bolts 53, and therefore an error may occur depending on how to tighten the bolts 53, making it difficult to attain a predetermined accuracy and requiring much time for the adjustment, thus leading to increase of cost.